Some compasses are designed with a housing which includes a neoprene expansion diaphragm that allows for expansion of the damping oil when the compass is subjected to heat which can occur when the compass is in the sun on the bridge of a ship. The diaphragm expands and contracts maintaining proper fluid pressure in the compass unit.
But in high altitude and/or low temperature conditions, air bubbles can still form in the compass. At high altitude conditions, the pressure outside the compass is reduced and therefore the diaphragm expands outward. When this happens, the volume inside the compass unit is increased and air is released from the oil creating an air bubble at the top of the upper compass bowl which may interfere with the ability to accurately read the compass heading and is cosmetically unappealing. It has also been found that air bubbles will form between the time the compass units are manufactured and sent to distributors and/or retailers.
Others have attempted to reduce or eliminate the formation of air bubbles with some apparent success. Johnson, for example, in U.S. Pat. No. 2,336,789, discloses a compass structure designed to prevent air bubble formation in the compass box. Included in the oil-filled compass box are hermetically sealed, gas filled, flexible tubes which contract and expand in response to increases and decreases in external pressure. Erickson, U.S. Pat. No. 1,491,593 discloses means, such as a compensating pocket within the compass housing which includes a flexible diaphragm that is free to flex in either direction in accordance with varying pressures exerted thereon and is therefore free to respond to the expansion or contraction of liquid within the compass bowl; the body of air or gas below the diaphragm acts to cushion movement of the diaphragm. Ekeberg, U.S. Pat. No. 2,136,970, discloses a compass which adjusts in response to temperature variations to prevent the formation of "vesicles" or leakage. The compass chamber is divided by a resilient aluminum plate into two sub-chambers. Both chambers are filled with oil under a vacuum and the aluminum plate adjusts itself to any changes in volume of the enclosed liquid caused by variations in temperature. Fowler, U.S. Pat. No. 4,004,348, discloses a housing surrounding a complete magnetic compass. The housing includes one resilient portion and is filled with damping fluid under pressure to provide an internal pressure higher than atmospheric. In this way, when the housing is subjected to high external pressures, the inner and outer pressure are equalized thereby minimizing the differential pressure on the housing itself. The focus of this invention is not the prevention of air bubbles, but the elimination of high strength materials for the compass housing. See column 1, lines 23-36.
Therefore, all of these methodologies and resulting structures require, to some extent, modifications to existing compass designs.